\documentclass[12pt]{article}
\usepackage[american]{babel}
\usepackage{siunitx}
\usepackage{booktabs, multirow, longtable, tabu}
\usepackage[onehalfspacing]{setspace}

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\usepackage{showframe}% to show page layout
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\begin{document}

\begin{longtabu} to \textwidth { X[1.5, l] X[1, l]
                               *{6}{S[table-format=2.3]}
                                }
\toprule
    & & \multicolumn{3}{c}{\textbf{def1}}   & \multicolumn{3}{c}{\textbf{def2}} \\
    \cmidrule(lr){3-5}\cmidrule(lr){6-8}
    & & {contrast} & {t} & {p}              & {contrast} & {t} & {p}            \\
\midrule
\endfirsthead
\toprule
    & & \multicolumn{3}{c}{\textbf{def1}}   & \multicolumn{3}{c}{\textbf{def2}} \\
    \cmidrule(lr){3-5}\cmidrule(lr){6-8}
    & & contrast & t & p                    & contrast & t & p \\
\midrule
\endhead
    \bottomrule
\multicolumn{8}{r}{continued on the next page} \\
\endfoot
\bottomrule
\endlastfoot
% end of tabu preamble
\multicolumn{8}{c}{\textbf{group 1}} \\
\midrule
\multirow{6}{*}{\textbf{design1}}
    & pos1  & 15.05 & 1.06 & 0.145 & -3.41 & -0.28 & 0.610 \\
    & pos2  & 9.20  & 0.65 & 0.258 & -4.02 & -0.32 & 0.627 \\
    & pos3  & 32.92 & 2.30 & 0.011 &  4.87 &  0.40 & 0.344 \\
    & pos4  & 6.46  & 0.69 & 0.247 &  3.94 &  0.45 & 0.327 \\
    & pos5  & 10.99 & 0.87 & 0.191 &  0.41 &  0.03 & 0.486 \\
    & pos6  & 36.70 & 2.20 & 0.014 & 30.20 &  1.94 & 0.026 \\
\midrule
\multirow{6}{*}{\textbf{design2}}  
    & pos1  & 6.51  & 0.87 & 0.192 & -2.79 & -0.41 & 0.661 \\
    & pos2  & 1.91  & 0.29 & 0.385 & -3.21 & -0.54 & 0.706 \\
    & pos3  & 16.49 & 2.52 & 0.006 & 4.21  &  0.73 & 0.233 \\
    & pos4  & 4.42  & 0.96 & 0.168 & 1.15  &  0.28 & 0.391 \\
    & pos5  & 0.13  & 0.02 & 0.491 & -0.26 & -0.05 & 0.521 \\
    & pos6  & 11.73 & 1.63 & 0.052 & 12.18 &  1.88 & 0.031 \\
\midrule
\multirow{6}{*}{\textbf{design3}}  
    & pos1  & 17.02 & 1.18 & 0.119 & -5.75 & -0.46 & 0.678 \\
    & pos2  & 11.08 & 0.77 & 0.220 & -3.85 & -0.30 & 0.618 \\
    & pos3  & 35.82 & 2.41 & 0.008 &  6.00 &  0.48 & 0.316 \\
    & pos4  & 9.59  & 0.96 & 0.170 &  1.78 &  0.19 & 0.423 \\
    & pos5  & 9.25  & 0.70 & 0.243 &  0.86 &  0.07 & 0.472 \\
    & pos6  & 34.91 & 1.99 & 0.024 & 32.28 &  2.04 & 0.021 \\
\midrule
\multicolumn{8}{c}{\textbf{group2}} \\
\midrule
\multirow{6}{*}{\textbf{design1}}  
    & pos1  & 16.98 & 1.07 & 0.142 & -10.16 & -0.70 & 0.759 \\
    & pos2  & 18.47 & 1.10 & 0.136 &  2.77  &  0.18 & 0.429 \\
    & pos3  & 15.28 & 1.23 & 0.110 & 16.18  &  1.41 & 0.079 \\
    & pos4  & 21.38 & 1.92 & 0.027 & -0.18  & -0.02 & 0.507 \\
    & pos5  & 30.56 & 2.54 & 0.006 & 16.49  &  1.50 & 0.067 \\
    & pos6  & 29.32 & 3.40 & 0.000 & -7.31  & -0.98 & 0.836 \\
\midrule
\multirow{6}{*}{\textbf{design2}}  
    & pos1  & 5.45  & 0.71 & 0.239 & -3.97 & -0.56 & 0.712 \\
    & pos2  & 9.89  & 1.48 & 0.070 & -0.84 & -0.14 & 0.554 \\
    & pos3  & 7.32  & 1.38 & 0.084 &  4.53 &  0.93 & 0.176 \\
    & pos4  & 10.15 & 2.04 & 0.021 & -0.86 & -0.19 & 0.575 \\
    & pos5  & 11.09 & 2.20 & 0.014 &  3.13 &  0.67 & 0.250 \\
    & pos6  & 9.31  & 1.88 & 0.030 & -3.42 & -0.75 & 0.774 \\
\midrule
\multirow{6}{*}{\textbf{design3}}  
    & pos1  & 16.54 & 1.06 & 0.145 & -7.81 & -0.55 & 0.709 \\
    & pos2  & 11.86 & 0.72 & 0.236 & -6.58 & -0.43 & 0.667 \\
    & pos3  & 15.75 & 1.29 & 0.099 & 14.89 &  1.32 & 0.093 \\
    & pos4  & 25.87 & 2.36 & 0.009 & -1.19 & -0.12 & 0.548 \\
    & pos5  & 30.73 & 2.59 & 0.005 & 14.17 &  1.31 & 0.096 \\
    &¸pos6  & 31.61 & 3.66 & 0.000 & -6.28 & -0.85 & 0.802 \\
\end{longtabu}

\end{document}
